The present invention relates to a method and apparatus for manufacturing a micromechanical device using a fluorine etchant source, for example xenon difluoride (XeF2). Xenon difluoride is a dry isotropic gas phase etchant, which provides a gentle etch for silicon at low temperature. Xenon difluoride is usually supplied in the form of colourless crystals which sublime without decomposition. The sublimation pressure for XeF2 is approximately 4 Torr.
A current method of manufacturing a micromechanical device uses the steps of anisotropic etching, mask removal, silicon oxide deposition, selective oxide etching, and isotropic etching of silicon with a SF6 plasma and oxide strip. However, this method requires many steps which results in long cycle times and a high cost. Other methods etch down to a buried oxide layer of silicon dioxide and selectively stop the anisotropic silicon etch at that interface. The silicon oxide is then selectively removed by HF to release the silicon structures. However, this method requires the use of wafers having a buried oxide layer which are very costly as well as additional equipment for HF based etching. Furthermore, liquid HF etching itself has considerable potential for stiction issues.
According to a first aspect of the present invention, here is provided a method of manufacturing a micromechanical device comprising the steps of:
(a) etching a substrate, having a mask thereon, through an opening in the mask to a desired depth to form a trench having a side wall and a base in the substrate;
(b) depositing a layer of a protecting substance on the exposed surfaces of the substrate and mask;
(c) selectively removing the protecting substance from the base; and
(d) etching the base using a fluorine-containing etchant.
The fluorine-containing etchant may be a gas or vapour used to etch the substrate in the absence of a plasma.
Preferably, the fluorine-containing etchant is XeF2. However, other vapours such as F2 or ClF3 (which will either etch or enhance the etch rate of silicon in the gas phase) as discussed in our co-pending British Patent Application No. 9904925.6 may be equally applied either as a replacement to XeF2, or in the form of a mixture, for example with XeF2. Thus, whilst the use of XeF2 is discussed below in detail, other gases could be used.
Conveniently, the mask may be provided with a plurality of openings therein.
The method may further comprise the step of removing the mask and remaining protecting substance. This removal may be by means of a plasma. The substrate is preferably formed of silicon.
In a preferred embodiment, the mask is deposited by a photolithographic process.
The substrate is preferably etched by substantially anisotropic etching. The anisotropic etching may conveniently be performed using the methods described in EP-A-0822582 and EP-A-0822584, the contents of which are incorporated herein by reference.
The use of XeF2 in a method and apparatus for etching a workpiece are disclosed in British Patent Application No. 9709659.8, the contents of which are incorporated herein by reference.
The protecting substance may be of the form CYFX (where X and Y may be any suitable value) such as CFX polymeric chains, where x may be 2. Thus, the protecting substance may be of the general formula n(CF2). One example may be cross-linked PTFE. The deposition may be performed by generating a plasma, for example an RF plasma, with an appropriate source gas, for example C4F8. In a preferred embodiment, a typical deposition thickness of the protecting substance is in the range of about 10 to 100 nm. Alternatively, the protecting substance may be of the form CYHX such as CHX polymeric chains, where x may be 2. Thus, it may be of the general formula n(CH2). In this case the source gas may be CH4, for example. Again X and Y may be any suitable value.
The protecting substance may be selectively removed from any or all of the surfaces other than the side wall and etching with the fluorine-containing etchant may take place on all resulting unprotected areas.
The protecting substance may selectively be removed by a plasma such as oxygen or (well known) mixtures with argon, helium, or nitrogen for example. The requirement for selective removal may be achieved by a suitable plasma process as known in the art.
The etching with the fluorine-containing etchant may be isotropic etching of the unprotected areas of the substrate.
According to a second aspect of the present invention, there is provided a device formed by the above method.
The device may be formed as a series of adjacent fingers separated by trenches. A fluorine-containing etchant, for example XeF2, may be used to undercut such that the trenches are in communication.
According to a third aspect of the present invention, there is provided an apparatus for manufacturing a micromechanical device comprising:
(a) means for etching a substrate, having a mask thereon, to a desired depth;
(b) means for depositing a layer of a protecting substance on the exposed surfaces of the substrate and mask;
(c) means for selectively removing the protecting substance; and
(d) means for providing a fluorine-containing etchant for etching the unprotected areas.
The means for depositing the protecting substance may be means for generating a plasma. The means for selectively removing the protecting substance preferably includes means for providing a suitable plasma.
In a preferred embodiment, the apparatus may further comprise means for removing the mask and the remaining protecting substance and this means may be a plasma source, preferably an oxygen plasma source.
Although the invention has been described above, it is to be understood that it includes any inventive combination of the features set out above or in the following description.